1. Field of the Invention
The present invention relates to a Bi (bismuth)-substituted rare earth iron garnet single crystal material that is a magnetic garnet material. The present invention also relates to a magnetooptical device utilizing a magnetooptical effect provided by the use of a magnetic garnet material and, more particularly, to a Faraday rotator.
2. Description of the Related Art
Conventional optical communication has been established by communication systems utilizing light having a single wavelength such as 1310 nm or 1550 nm. Since optical isolators which are optically passive components used in conventional optical communication systems are used at a single wavelength as described above, Faraday rotators which are magnetooptical devices forming a part of optical isolators are also developed such that they exhibit good characteristics at a single wavelength such as 1310 nm or 1550 nm. For example, Japanese examined patent publication (KOKOKU) No. H3-69847 (1991) has disclosed a Bi-substituted rare earth iron garnet single crystal material which includes Tb (terbium). Temperature characteristics of a Faraday rotator can be improved by fabricating it using the magnetic garnet material. For this reason, optical isolators utilizing Faraday rotators primarily constituted by Tb are widely used in optical communication systems.
The recent spread of the internet has dramatically increased the amount of communication over communication lines. Proposals have been made for optical wavelength division multiplex communication systems (hereinafter referred to as xe2x80x9cWDM communication systemsxe2x80x9d) in which a plurality of optical signals having different wavelengths are simultaneously transferred over a single optical fiber. An optical amplifier used in a WDM communication system directly amplifies an optical signal using an erbium-doped fiber as an amplifying medium. In the case of a WDM communication system, for example, a plurality of optical signals having different wavelengths within the L-waveband (wavelengths in the range from 1570 nm to 1620 nm) are transferred.
Under such circumstances, optically passive components such as optical isolators, optical attenuators and composite optical modules must have high magnetooptical characteristics in wavebands higher than the wavelength of 1550 nm according to the prior art. However, Faraday rotators fabricated using a Bi-substituted rare earth iron garnet single crystal including Tb have a significant insertion loss at wavebands longer than 1550 nm. Therefore, optically passive components constituted by a Faraday rotator including Tb have had a great insertion loss in the case of light in wavebands longer than 1550 nm.
It is therefore difficult to satisfy an insertion loss characteristic of 0.1 dB or less required in the L-waveband used for WDM communication systems with Faraday rotators which are primarily constituted by Tb.
The output of a light source must therefore be increased in order to maintain a predetermined quantity of light in an optical communication system, and this results in a problem in that the cost of the optical communication system is increased.
Further, since the Faraday rotation coefficient (deg./xcexcm) decreases as the wavelength of light increases, a Faraday rotator fabricated using a Bi-substituted rare earth iron garnet single crystal material must have a large thickness in order to achieve a Faraday angle of 45 deg. required for the same. For this reason, the thickness required for a Faraday rotator of an optical isolator used in a waveband such as the L-waveband for WDM communication systems which is longer than conventionally used wavelengths is greater than that of rotators used at a single wavelength of 1550 nm. This has resulted in a problem in that a great number of cracks occur during the growth of a single crystal film or during lapping of the same into a Faraday rotator, thereby causing a reduction of yield.
It is an object of the invention to provide a magnetic garnet material which is less likely to crack during the growth of a single crystal film or during lapping of the same.
It is another object of the invention to provide a magnetooptical device which defines a Faraday rotation angle xcex8 expressed by 44 deg.xe2x89xa6xcex8xe2x89xa646 deg. when light having a predetermined wavelength xcex (1570 nmxe2x89xa6xcexxe2x89xa61620 nm) impinges thereupon, and which is less likely to crack during processing in order to permit the suppression of a reduction of yield.
The above-described object is achieved by a magnetic garnet material characterized in that it is expressed by a general formula: BiaM13xe2x88x92aFe5xe2x88x92bM2bO12.
M1 is at least one kind of element that is selected from among Y, La, Eu, Gd, Ho, Yb, Lu and Pb. M2 is at least one kind of element that is selected from among Ga, Al, Ti, Ge, Si and Pt; and xe2x80x9caxe2x80x9d and xe2x80x9cbxe2x80x9d satisfy 1.0xe2x89xa6axe2x89xa61.5 and 0xe2x89xa6bxe2x89xa60.5, respectively.
A magnetic garnet material according to the invention as described above is characterized in that the material is grown by liquid phase epitaxial growth method.
The above-described object is achieved by a magnetooptical device has a Faraday rotation angle xcex8 expressed by 44 deg.xe2x89xa6xcex8xe2x89xa646 deg. when light having a predetermined wavelength xcex (1570 nmxe2x89xa6xcexxe2x89xa61620 nm) impinges thereupon, characterized in that it is formed of a magnetic garnet material according the invention as described above.
A magnetooptical device according to the invention as described above is characterized in that it has an insertion loss of 0.1 dB or less when light having the wavelength xcex impinges thereupon.